1. Field of the Invention
The present invention relates to techniques for performing formation evaluation of a subterranean formation by a downhole tool positioned in a wellbore penetrating the subterranean formation. More particularly, the present invention relates to techniques for reducing the contamination of formation fluids drawn into and/or evaluated by the downhole tool.
2. Background of the Related Art
Wellbores are drilled to locate and produce hydrocarbons. A downhole drilling tool with a bit at and end thereof is advanced into the ground to form a wellbore. As the drilling tool is advanced, a drilling mud is pumped through the drilling tool and out the drill bit to cool the drilling tool and carry away cuttings. The fluid exits the drill bit and flows back up to the surface for recirculation through the tool. The drilling mud is also used to form a mudcake to line the wellbore.
During the drilling operation, it is desirable to perform various evaluations of the formations penetrated by the wellbore. In some cases, the drilling tool may be provided with devices to test and/or sample the surrounding formation. In some cases, the drilling tool may be removed and a wireline tool may be deployed into the wellbore to test and/or sample the formation. In other cases, the drilling tool may be used to perform the testing or sampling. These samples or tests may be used, for example, to locate valuable hydrocarbons.
Formation evaluation often requires that fluid from the formation be drawn into the downhole tool for testing and/or sampling. Various devices, such as probes, are extended from the downhole tool to establish fluid communication with the formation surrounding the wellbore and to draw fluid into the downhole tool. A typical probe is a circular element extended from the downhole tool and positioned against the sidewall of the wellbore. A rubber packer at the end of the probe is used to create a seal with the wellbore sidewall. Another device used to form a seal with the wellbore sidewall is referred to as a dual packer. With a dual packer, two elastomeric rings expand radially about the tool to isolate a portion of the wellbore therebetween. The rings form a seal with the wellbore wall and permit fluid to be drawn into the isolated portion of the wellbore and into an inlet in the downhole tool.
The mudcake lining the wellbore is often useful in assisting the probe and/or dual packers in making the seal with the wellbore wall. Once the seal is made, fluid from the formation is drawn into the downhole tool through an inlet by lowering the pressure in the downhole tool. Examples of probes and/or packers used in downhole tools are described in U.S. Pat. Nos. 6,301,959; 4,860,581; 4,936,139; 6,585,045; 6,609,568 and 6,719,049 and US Patent Application No. 2004/0000433.
The collection and sampling of underground fluids contained in subsurface formations is well known. In the petroleum exploration and recovery industries, for example, samples of formation fluids are collected and analyzed for various purposes, such as to determine the existence, composition and producibility of subsurface hydrocarbon fluid reservoirs. This aspect of the exploration and recovery process can be crucial in developing drilling strategies and impacts significant financial expenditures and savings.
To conduct valid fluid analysis, the fluid obtained from the subsurface formation should possess sufficient purity, or be virgin fluid, to adequately represent the fluid contained in the formation. As used herein, and in the other sections of this patent, the terms “virgin fluid”, “acceptable virgin fluid” and variations thereof mean subsurface fluid that is pure, pristine, connate, uncontaminated or otherwise considered in the fluid sampling and analysis field to be sufficiently or acceptably representative of a given formation for valid hydrocarbon sampling and/or evaluation.
Various challenges may arise in the process of obtaining virgin fluid from subsurface formations. Again with reference to the petroleum-related industries, for example, the earth around the borehole from which fluid samples are sought typically contains contaminates, such as filtrate from the mud utilized in drilling the borehole. This material often contaminates the virgin fluid as it passes through the borehole, resulting in fluid that is generally unacceptable for hydrocarbon fluid sampling and/or evaluation. Such fluid is referred to herein as “contaminated fluid.” Because fluid is sampled through the borehole, mudcake, cement and/or other layers, it is difficult to avoid contamination of the fluid sample as it flows from the formation and into a downhole tool during sampling. A challenge thus lies in minimizing the contamination of the virgin fluid during fluid extraction from the formation.
FIG. 1 depicts a subsurface formation 16 penetrated by a wellbore 14. A layer of mud cake 15 lines a sidewall 17 of the wellbore 14. Due to invasion of mud filtrate into the formation during drilling, the wellbore is surrounded by a cylindrical layer known as the invaded zone 19 containing contaminated fluid 20 that may or may not be mixed with virgin fluid. Beyond the sidewall of the wellbore and surrounding contaminated fluid, virgin fluid 22 is located in the formation 16. As shown in FIG. 1, contaminates tend to be located near the wellbore wall in the invaded zone 19.
FIG. 2 shows the typical flow patterns of the formation fluid as it passes from subsurface formation 16 into a downhole tool 1. The downhole tool 1 is positioned adjacent the formation and a probe 2 is extended from the downhole tool through the mudcake 15 to the sidewall 17 of the wellbore 14. The probe 2 is placed in fluid communication with the formation 16 so that formation fluid may be passed into the downhole tool 1. Initially, as shown in FIG. 1, the invaded zone 19 surrounds the sidewall 17 and contains contamination. As fluid initially passes into the probe 2, the contaminated fluid 20 from the invaded zone 19 is drawn into the probe with the fluid thereby generating fluid unsuitable for sampling. However, as shown in FIG. 2, after a certain amount of fluid passes through the probe 2, the virgin fluid 22 breaks through and begins entering the probe. In other words, a more central portion of the fluid flowing into the probe gives way to the virgin fluid, while the remaining portion of the fluid is contaminated fluid from the invasion zone. The challenge remains in adapting to the flow of the fluid so that the virgin fluid is collected in the downhole tool during sampling.
Formation evaluation is typically performed on fluids drawn into the downhole tool. Techniques currently exist for performing various measurements, pretests and/or sample collection of fluids that enter the downhole tool. Various methods and devices have been proposed for obtaining subsurface fluids for sampling and evaluation. For example, U.S. Pat. No. 6,230,557 to Ciglenec et al., U.S. Pat. No. 6,223,822 to Jones, U.S. Pat. No. 4,416,152 to Wilson, U.S. Pat. No. 3,611,799 to Davis and International Pat. App. Pub. No. WO 96/30628 have developed certain probes and related techniques to improve sampling. However, it has been discovered that when the formation fluid passes into the downhole tool, various contaminants, such as wellbore fluids and/or drilling mud, may enter the tool with the formation fluids. These contaminates may affect the quality of measurements and/or samples of the formation fluids. Moreover, contamination may cause costly delays in the wellbore operations by requiring additional time for more testing and/or sampling. Additionally, such problems may yield false results that are erroneous and/or unusable. Other techniques have been developed to separate virgin fluids during sampling. For example, U.S. Pat. No. 6,301,959 to Hrametz et al. and discloses a sampling probe with two hydraulic lines to recover formation fluids from two zones in the borehole. Borehole fluids are drawn into a guard zone separate from fluids drawn into a probe zone. Despite such advances in sampling, there remains a need to develop techniques for fluid sampling to optimize the quality of the sample and efficiency of the sampling process.
It is, therefore, desirable that the formation fluid entering into the downhole tool be sufficiently ‘clean’ or ‘virgin’ for valid testing. In other words, the formation fluid should have little or no contamination. Attempts have been made to eliminate contaminates from entering the downhole tool with the formation fluid. For example, as depicted in U.S. Pat. No. 4,951,749, filters have been positioned in probes to block contaminates from entering the downhole tool with the formation fluid. Additionally, as shown in U.S. Pat. No. 6,301,959 to Hrametz, a probe is provided with a guard ring to divert contaminated fluids away from clean fluid as it enters the probe.
Despite the existence of techniques for performing formation evaluation and for attempting to deal with contamination, there remains a need to manipulate the flow of fluids through the downhole tool to reduce contamination as it enters and/or passed through the downhole tool. It is desirable that such techniques are capable of diverting contaminants away from clean fluid. It is further desirable that such techniques be capable of one of more of the following, among others: analyzing the fluid passing through the flowlines, selectively manipulating the flow of fluid through the downhole tool, responding to detected contamination, removing contamination and/or providing flexibility in handling fluids in the downhole tool.
In considering existing technology for the collection of subsurface fluids for sampling and/or evaluation, there remains a need for techniques capable of providing one or more, among others, of the following attributes: the ability to selectively collect virgin fluid apart from contaminated fluid; the ability to separate virgin fluid from contaminated fluid; the ability to optimize the quantity and/or quality of virgin fluid extracted from the formation for sampling; the ability to adjust the flow of fluid according to the sampling needs; the ability to control the sampling operation manually and/or automatically and/or on a real-time basis.
Techniques have also been developed to evaluate fluid passing through the tool to determine contamination levels. In some cases, techniques and mathematical models have been developed for predicting contamination for a merged flowline. See, for example, Published PCT Application No. WO 2005065277 and PCT Application No. 00/50876, the entire contents of which are hereby incorporated by reference. Techniques for predicting contamination levels and determining cleanup times are described in P. S. Hammond, “One or Two Phased Flow During fluid Sampling by a Wireline Tool,” Transport in Porous Media, Vol. 6, p. 299-330 (1991), the entire contents of which are hereby incorporated by reference. Hammond describes a semi-empirical technique for estimating contamination levels and cleanup time of fluid passing into a downhole tool through a single flowline.
While techniques have been developed for contamination monitoring, such techniques relate to single flowline applications. It is desirable to provide contamination monitoring techniques applicable to multi-flowline operations. It is further desirable that such techniques provide one or more of the following capabilities: analyzing the fluid flow to detect contamination levels, estimate time to clean up contamination, calibrate flowline measurements, cross-check flowline measurements, selectively combine and/or separate flowlines, determining contamination levels and compare flowline data to known values. To this end, the present invention seeks to optimize the formation evaluation process.